The invention relates to printed circuit boards and more particularly, to a printed circuit board of an electrical connector having an insulative coating covering the conductive circuitry and the side walls of contact receiving apertures formed therein.
Solder is a tin-lead alloy which has been and is presently, utilized in one respect or another to effect substantially all electrical interconnections upon printed circuit boards. In fact, in certain printed circuit board applications, it is even desirable to coat exposed copper circuitry patterns with solder even when no additional electrical connection is to be made. This fabrication step protects the circuitry from oxidation and other environmental effects and may facilitate subsequent electrical assembly. Although subsequent solder operations are not always necessary for assembly of modern printed circuit boards, the almost universal acceptance of solder in the electrical connector industry has precipitated its continued use. More recently, however, it has been recognized that the prerequisite heat of solder application, as well as certain metallurgical aspects of the alloy itself, may be disadvantageous from a product reliability standpoint. For such reasons, the specific assembly requirements and fabrication techniques of printed circuit boards and connectors have come under close scrutiny.
Certain prior art approaches to printed circuit boards and electrical connector assemblies have included press fitting contacts directly into plated through holes of a mounting substrate to support the contacts and hold them rigidly in a fixed configuration. This press fit approach is in contrast to that of inserting a contact into a plated through hole in a mounting substrate and soldering it therein. Although solder may be present in both instances, in the latter, the reflowed solder is the primary interfacing element of a solid electrical-mechanical interconnection. In the press fit aperture, solder functions in the main, as an environmental coating. Of course the advantages of the press fit assembly are readily recognizable in that it is virtually impossible to remove a contact structurally adhered by solder to the mounting substrate for repair without reheating the solder. The press fitted pin may be pushed out whether or not there is solder in the hole, facilitating the insertion of a new pin into the original hole. The malleable solder lining of the press fit hole is generally displaced or deformed, as is the conductive, plated through lining thereof, during press fitting of the rigid pin therein. Angular edges on the press fit pin are particularly effective in accomplishing this result. In such an assembly it may be seen that the function of the solder in the plated through hole is limited to that of coating and protecting the uncontacted, or exposed, side walls of the hole. Such an assembly, of the card edge connector type, is described in U.S. Pat. No. 3,671,917, issued to John P. Ammon and Frederick T. Inacker on June 20, 1972, and assigned to the assignee of the present invention.
A trend in the state of the art development of the substrate mounted connector is the use of structures which include an insulator removable from around an array of contacts rigidly mounted in a substrate through press fit interengagement. A principal reason for removable layover-insulators is repairability. In certain instances the removable insulator may even serve as the initial contact holding fixture and press fitting tool. The resulting printed circuit board connector can therein be assembled from independent, prefabricated subassemblies without the application of solder or the heat therefrom. The only solder which may be present on the assembly is that which is applied to the printed circuit board in its fabrication stage as a coating for the circuitry on the board and in the contact receiving apertures. Such an assembly is shown and described in U.S. Pat. No. 4,045,868 issued Sept. 6, 1977 to Preston Ammon, et al, and assigned to the assignee of the present invention.
In certain prior art structures, the application of the solder coating to substantial portions of the circuitry pattern of the printed circuit board has, in itself, been eliminated. Coating substitutes have, in such structures, been comprised of epoxies and related dielectric copolymers which may be applied in a liquid state and allowed to harden into a sealed encapsulation surface. Often the coating is applied by a silkscreen process, allowing selectivity in pattern applications. The coating is, however, specifically not applied to the circuitry in and around the plated through holes of the printed circuit board before the contacts are inserted since the coating is insulative. These regions of electrical interconnection are again coated with conductive molten solder as in previous applications. For this reason, the coating of insulative epoxy in this selective configuration is commonly referred to as a solder mask. The utility and effectiveness of a solder mask are most clearly manifested when the contact receiving apertures are press fitted with contact terminals. In this manner the printed circuit board is not subjected to the heat of soldering for securing the contacts therein. The portions of the printed circuit board in and around the plated through holes comprise the only area thereof coated with solder and therefore the only areas necessitating the application heat. Unfortunately, it is impractical to heat only the region of contact aperture circuitry when applying solder to a printed circuit board with or without a solder mask, and therefore the entire board is exposed to the molten material for anywhere from two to four seconds. The melting point of most tin-lead alloys is 300.degree. C. and such a temperature is, to an established degree, destructive to the board fibers and the assembly thereof. The solder application temperature may thus be seen to be a controlling parameter for selection of most printed circuit board materials and methods of assembly.
It would be an advantage, therefore, to provide a printed circuit board and electrical connector which could be fabricated and assembled without solder and the requisite heat thereof. The apparatus of the present invention is especially adapted for just such a purpose. The printed circuit board is fabricated to the construction stage wherein conductive circuitry interconnects plated through holes found therein. The board is then coated with a dielectric mask which seals substantially all of the outer surfaces of circuitry, including the plated through holes, from the environment. The plated through holes are adapted for receiving contact terminals press fitted therein. The contacts are provided with angular edge portions which effectively penetrate and deform the dielectric mask away from that portion of the plated through hole brought to bear against the contact. The untouched portion of the hole remains coated and sealed from the environment while a rigid electrical and mechanical connection is effected. In this manner, solder is eliminated and the requisite heat thereof may be removed as a design parameter.